Method and device for manufacturing a hologram recording medium

ABSTRACT

A method and device for manufacturing a hologram recording medium. Arrangements are made to enable different original images to be reproduced upon observation from different positions and yet enable reproduced images of high resolution to be obtained.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/811,599 filed Jun. 11, 2007, which claimed the benefit of JapanesePatent Application: 2006/164332 filed Jun. 14, 2006, the contents ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method and a device for manufacturinga hologram recording medium and particularly relates to arts formanufacturing a hologram recording medium that is arranged to reproducedifferent original images when observed from different positions.

Holograms have come to be widely used in applications for preventingcounterfeiting of cash vouchers and credit cards. Normally, a regiononto which a hologram is to be recorded is set up in a portion of amedium to be subject to counterfeiting prevention, and a hologram of athree-dimensional image, etc., is recorded inside this region.

With many holograms that are currently utilized commercially, anoriginal image is recorded onto a medium in the form of interferencefringes by an optical method. That is, a method is employed in which anobject that forms an original image is prepared and light from theobject and a reference light are guided by a lens or other opticalsystem to a recording surface, coated with a photosensitizing agent, toform interference fringes on the recording surface. Although thisoptical method requires an optical system of considerably high precisionto obtain a clear image, it is the most direct method for obtaining ahologram and is the most widely practiced method in industry.

Meanwhile, methods for preparing a hologram by forming interferencefringes on a recording surface by computation using a computer have cometo be known recently, and a hologram prepared by such a method isgenerally referred to as a “computer generated hologram (CGH)” or simplyas a “computer hologram.” A computer hologram is obtained by simulatingan optical interference fringe generating process on a computer, and anentire process of generating an interference fringe pattern is carriedout in the form of computation on the computer. Upon obtaining imagedata of an interference fringe pattern by such a computation, physicalinterference fringes are formed on an actual medium based on the imagedata. As a specific example, a method, with which image data of aninterference fringe pattern prepared by a computer are provided to anelectron beam printer and physical interference fringes are formed byscanning an electron beam across a medium, has been put to practicaluse.

With a hologram recording medium, an original image can be recordedthree-dimensionally and the original image can be observed fromdifferent angles by changing the viewpoint position. Thus, a majorcharacteristic of a hologram recording medium is that athree-dimensional image can be recorded on a flat surface. Also,recently, hologram recording media, with a further characteristic that acompletely different original image is reproduced when observed from adifferent angle, are being utilized commercially. For example, JapanesePatent Laid-open Publication No. 2001-109362A discloses a method thatemploys a computer generating hologram method to manufacture a hologramrecording medium with which different original images can be reproducedby changing the viewpoint position.

As mentioned above, methods for manufacturing a hologram recordingmedium, with which different original images can be reproduced whenobserved from different positions, are already as known as conventionalarts. However, because the basic principle of the conventional methodsis to set up a plurality of regions on a hologram recording surface andrecord a different original image on each individual region, there isthe problem that the reproduced images are lowered in resolution.

For example, the abovementioned Patent Document discloses a method inwhich a hologram recording surface is partitioned into a plurality ofstrip-like regions, each strip-like region is associated with oneoriginal image among a plurality of mutually different original images,and on a single strip-like region, only the one original image that isassociated with the strip-like region is recorded. Specifically, in acase where three original images are to be recorded, a recording methodis employed in which a first original image is recorded on a 1st, 4th,7th, 10th strip-like regions, etc., a second original image is recordedon a 2nd, 5th, 8th, 11th strip-like regions, etc., and a third originalimage is recorded on a 3rd, 6th, 9th, 12th strip-like regions, etc. Inthis case, each of the three original images is recorded in the form ofinterference fringes and by differing the direction of the referencelight according to each original image in this process, a specificoriginal image is made to be reproduced upon observation from a specificposition.

However with the above example, because, for example, the first originalimage is recorded only on the 1st, 4th, 7th, 10th strip-like regions,etc., and information on the first original image are left out from the2nd, 3rd, 5th, 6th, 8th, 9th, 11th, 12th strip-like regions, etc., theresolution of the reproduced image is reduced to ⅓ that of the original.Thus, as long as the principle of recording a different original imageon each individual region is employed, the problem of lowering theresolution of the reproduced image occurs.

SUMMARY OF THE INVENTION

Thus, an object of the present invention is to provide a method and adevice for manufacturing a hologram recording medium, with whichdifferent original images are reproduced when observed from differentpositions and yet with which reproduced images of high resolution can beobtained.

(1) The first feature of the invention resides in a method formanufacturing a hologram recording medium that has an arrangement bywhich different original images are reproduced when observed fromdifferent positions, the hologram recording medium manufacturing methodcomprising:

an original image preparation step of preparing a plurality N oforiginal images, each as a set of unit light sources positioned in athree-dimensional coordinate system;

a recording plane setting step of setting a predetermined recordingplane in the three-dimensional coordinate system;

a reference light setting step of setting a predetermined referencelight in the three-dimensional coordinate system;

an observation region setting step of setting a plurality N ofobservation regions in the three-dimensional coordinate system;

a pattern computation step of computing an interference fringe pattern,formed on the recording plane, based on object light components, emittedfrom the individual unit light sources constituting the respectiveoriginal images, and the reference light; and

a pattern forming step of forming the interference fringe pattern on aphysical medium; and

wherein in the pattern computation step, computations that each takesinto account only light components, which, among the object lightcomponents from the unit light sources belonging to an i-th (i=1, 2, . .. , N) original image, reach an i-th (i=1, 2, . . . , N) observationregion, are performed.

(2) The second feature of the invention resides in a hologram recordingmedium manufacturing method according to the first feature, wherein

in the pattern computation step, a synthetic object light is determinedby synthesizing the object light components emitted from the individualunit light sources constituting the respective original images, and aninterference fringe pattern that is obtained on the recording plane byinterference of the synthetic object light and the reference light iscomputed.

(3) The third feature of the invention resides in a hologram recordingmedium manufacturing method according to the first or second feature,wherein

in the pattern forming step, the interference fringe pattern, obtainedby the pattern computation step, is converted into a binary imagepattern and the binary image pattern is formed on a physical medium.

(4) The fourth feature of the invention resides in a method formanufacturing a hologram recording medium that has an arrangement bywhich different original images are reproduced when observed fromdifferent positions, the hologram recording medium manufacturing methodcomprising:

an original image preparation step of preparing a plurality N oforiginal images, each as a set of unit light sources positioned in athree-dimensional coordinate system;

a recording plane setting step of setting a predetermined recordingplane in the three-dimensional coordinate system;

an observation region setting step of setting a plurality N ofobservation regions in the three-dimensional coordinate system;

a pattern computation step of computing a complex amplitude patternformed on the recording plane by synthesis of object light componentsemitted from the individual unit light sources constituting therespective original images; and

a pattern forming step of forming the complex amplitude pattern on aphysical medium; and

wherein in the pattern computation step, computations that each takesinto account only light components, which, among the object lightcomponents from the unit light sources belonging to an i-th (i=1, 2, . .. , N) original image, reach an i-th (i=1, 2, . . . , N) observationregion, are performed.

(5) The fifth feature of the invention resides in a hologram recordingmedium manufacturing method according to the fourth feature, wherein

in the pattern computation step, a plurality of computation points aredefined discretely on the recording plane and an amplitude and a phaseof a synthetic object light at a predetermined sampling time point isdetermined for each of computation point positions to determine thecomplex amplitude pattern as a discrete distribution of amplitudes andphases.

(6) The sixth feature of the invention resides in a hologram recordingmedium manufacturing method according to the fifth feature, wherein

in the pattern forming step, a cell, formed of a three-dimensionalstructure, is positioned at each individual computation position andinformation of an amplitude and a phase concerning the computation pointposition corresponding to each individual cell are recorded in thethree-dimensional structure of the cell.

(7) The seventh feature of the invention resides in a hologram recordingmedium manufacturing method according to the first to sixth features,wherein

in the original image preparation step, a plurality of original imagesthat are positioned so as to partially overlap spatially are prepared.

(8) The eighth feature of the invention resides in a hologram recordingmedium manufacturing method according to the first to seventh features,wherein

in the observation region setting step, the plurality N of observationregions are set to be regions that are spatially exclusive with respectto each other.

(9) The ninth feature of the invention resides in a hologram recordingmedium manufacturing method according to the first to seventh features,wherein

in the observation region setting step, a portion or all of theplurality N of observation regions are set to be regions that partiallyoverlap spatially with another observation region.

(10) The tenth feature of the invention resides in a hologram recordingmedium manufacturing method according to the first to seventh features,wherein

in the observation region setting step, a portion or all of theplurality N of observation regions are set to be regions that spatiallymatch another observation region completely.

(11) The eleventh feature of the invention resides in a hologramrecording medium manufacturing method according to the first to tenthfeatures, wherein

point light sources or collections of point light sources are used asthe unit light sources, and the object light is defined as a sphericalwave that is emitted radially from each point light source or as asynthetic wave of such spherical waves.

(12) The twelfth feature of the invention resides in a hologramrecording medium manufacturing method according to the first to tenthfeatures, wherein

segment light sources are used as unit light sources, and object lightcomponents, each with a wavefront formed of a side surface of acylindrical column having a segment light source as a central axis,which propagate in a direction perpendicular to the central axis, aredefined.

(13) The thirteenth feature of the invention resides in a hologramrecording medium manufacturing method according to the first to twelfthfeatures, wherein

in the observation region setting step, each individual observationregion is set as a plane, a curved surface, or a three-dimensional bodyin the three-dimensional coordinate system.

(14) The fourteenth feature of the invention resides in a hologramrecording medium manufacturing method according to the first tothirteenth features, wherein

in the pattern computation step, the computation is performed uponpartitioning the three-dimensional space into a plurality M ofplate-like spaces by slicing by a plurality of mutually parallel planesand by taking into account only light components, which, among theobject light components from the unit light sources in a j-th (j=1, 2, .. . , M) plate-like space and belonging to an i-th (i=1, 2, . . . , N)original image, reach an i-th (i=1, 2, . . . , N) observation region andreach the recording plane only through an interior of the j-th (j=1, 2,. . . , M) plate-like space.

(15) The fifteenth feature of the invention resides in a method formanufacturing a hologram recording medium that has an arrangement bywhich different original images are reproduced when observed fromdifferent positions, the hologram recording medium manufacturing methodcomprising:

an original image preparation step of preparing a plurality of originalimages, each as a set of unit light sources positioned in athree-dimensional coordinate system;

a recording plane setting step of setting a predetermined recordingplane in the three-dimensional coordinate system;

a reference light setting step of setting a predetermined referencelight in the three-dimensional coordinate system;

an observation region setting step of setting a plurality of observationregions in the three-dimensional coordinate system;

a pattern computation step of computing an interference fringe pattern,formed on the recording plane, based on object light components, emittedfrom the individual unit light sources constituting the respectiveoriginal images, and the reference light; and

a pattern forming step of forming the interference fringe pattern on aphysical medium; and

wherein in the pattern computation step, computations that each takesinto account only light components, which, among the object lightcomponents from the unit light sources, propagate toward a uniqueobservation region set in accordance with the original image to whichthe unit light sources belong, are performed.

(16) The sixteenth feature of the invention resides in a method formanufacturing a hologram recording medium that has an arrangement bywhich different original images are reproduced when observed fromdifferent positions, the hologram recording medium manufacturing methodcomprising:

an original image preparation step of preparing a plurality of originalimages, each as a set of unit light sources positioned in athree-dimensional coordinate system;

a recording plane setting step of setting a predetermined recordingplane in the three-dimensional coordinate system;

an observation region setting step of setting a plurality of observationregions in the three-dimensional coordinate system;

a pattern computation step of computing a complex amplitude patternformed on the recording plane by synthesis of object light componentsemitted from the individual unit light sources constituting therespective original images; and

a pattern forming step of forming the complex amplitude pattern on aphysical medium; and

wherein in the pattern computation step, computations that each takesinto account only light components, which, among the object lightcomponents from the unit light sources, propagate toward a uniqueobservation region set in accordance with the original image to whichthe unit light sources belong, are performed.

(17) The seventeenth feature of the invention resides in a hologramrecording medium which is manufactured by the manufacturing methodaccording to the first to sixteenth features.

(18) The eighteenth feature of the invention resides in a device formanufacturing a hologram recording medium that has an arrangement bywhich different original images are reproduced when observed fromdifferent positions, the hologram recording medium manufacturing devicecomprising:

an original image storage unit, storing a plurality N of originalimages, each as data indicating a set of unit light sources positionedin a three-dimensional coordinate system;

a recording plane setting unit, setting a predetermined recording planein the three-dimensional coordinate system;

a reference light setting unit, setting a predetermined reference lightin the three-dimensional coordinate system;

an observation region setting unit, setting a plurality N of observationregions in the three-dimensional coordinate system;

a pattern computation unit, computing an interference fringe pattern,formed on the recording plane, based on object light components, emittedfrom the individual unit light sources constituting the respectiveoriginal images, and the reference light; and

a pattern forming unit, forming an interference fringe pattern on aphysical medium; and

wherein the pattern computation unit performs computations that eachtakes into account only light components, which, among the object lightcomponents emitted from the unit light sources belonging to an i-th(i=1, 2, . . . , N) original image, reach an i-th (i=1, 2, . . . , N)observation region.

(19) The nineteenth feature of the invention resides in a device formanufacturing a hologram recording medium that has an arrangement bywhich different original images are reproduced when observed fromdifferent positions, the hologram recording medium manufacturing devicecomprising:

an original image storage unit, storing a plurality N of originalimages, each as data indicating a set of unit light sources positionedin a three-dimensional coordinate system;

a recording plane setting unit, setting a predetermined recording planein the three-dimensional coordinate system;

an observation region setting unit, setting a plurality N of observationregions in the three-dimensional coordinate system;

a pattern computation unit, computing a complex amplitude pattern formedon the recording plane by synthesis of object light components emittedfrom the individual unit light sources constituting the respectiveoriginal images; and

a pattern forming unit, forming the complex amplitude pattern on aphysical medium; and

wherein the pattern computation unit performs computations that eachtakes into account only light components, which, among the object lightcomponents emitted from the unit light sources belonging to an i-th(i=1, 2, . . . , N) original image, reach an i-th (i=1, 2, . . . , N)observation region.

(20) The twentieth feature of the invention resides in a program thatmakes a computer function as the original image storage unit, therecording plane setting unit, the reference light setting unit, theobservation region setting unit, and the pattern computation unit of thehologram recording medium manufacturing device according to theeighteenth feature or a program that makes a computer function as theoriginal image storage unit, the recording plane setting unit, theobservation region setting unit, and the pattern computation unit of thehologram recording medium manufacturing device according to thenineteenth feature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of modes of observing a hologram recordingmedium manufactured by a method according to the present invention.

FIGS. 2A and 2B show plan views of two modes of observing the hologramrecording medium shown in FIG. 1.

FIG. 3 is a flowchart of a basic procedure of the hologram recordingmedium manufacturing method according to the present invention.

FIGS. 4A and 4B show perspective views of an example of two originalimages prepared in “S1: Original image preparation step” in theflowchart of FIG. 3.

FIG. 5 is an upper view of an example of original images Ia and Ib,prepared in “S1: Original image preparation step,” and a recording plane20, a reference light R, and observation regions Oa and Ob, respectivelyset in “S2: Recording plane setting step,” “S3: Reference light settingstep,” and “S4: Observation region setting step” in the flowchart ofFIG. 3.

FIG. 6 is a perspective view of concepts of a computation processperformed in “S5: Pattern computation step” in the flowchart of FIG. 3.

FIG. 7 is an upper view of computation-incorporated light componentsamong light components emitted from respective point light sourcesconstituting the original image Ia.

FIG. 8 is an upper view of computation-incorporated light componentsamong light components emitted from respective point light sourcesconstituting the original image Ib.

FIG. 9 is a sectional view of concepts of partitioning athree-dimensional space into a plurality of plate-like spaces andcomputing an interference fringe pattern using computation-incorporatedlight components, to which is applied a weighting condition of lightcomponents, which, among those emitted from the respective point lightsources, reach the recording plane upon propagating through only asingle plate-like space.

FIGS. 10A and 10B are diagrams of concepts of preparing a segment lightsource PP based on a point light source P.

FIG. 11 is a perspective view of an example of defining an object light,with a wavefront of the form of a side surface of a cylindrical column,for a segment light source QQ.

FIG. 12 is a side view of an example of a method for computing aninterference fringe pattern for an original image constituted of segmentlight sources.

FIG. 13 is a block diagram of a basic arrangement of a hologramrecording medium manufacturing device according to the presentinvention.

FIG. 14 is a block diagram of another basic arrangement of a hologramrecording medium manufacturing device according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention shall now be described based on the illustratedembodiments.

Section 1. Basic Embodiment of the Present Invention

A hologram recording medium manufacturing method according to thepresent invention shall first be described based on a basic embodiment.FIG. 1 is a perspective view of modes of observing a hologram recordingmedium 10 manufactured by the method according to the present invention.The hologram recording medium 10 shown here as an example is areflection type recording medium and as illustrated, a reproductionimage is obtained by observation from a front side while a reproductionillumination light Lr is illuminated from the front side. The hologramrecording medium has the characteristic that mutually differentreproduction images are observed upon observation from a viewpoint E1and upon observation from a viewpoint E2. As a matter of course, themethod according to the present invention is not restricted to themanufacture of a reflection type recording medium, and a transmissiontype recording medium, with which observation is performed from thefront side while the reproduction illumination light Lr is illuminatedfrom the back side, can also be prepared.

With the specific example shown here, whereas upon observation from theviewpoint E1, a first reproduction image A (an image of a cylindricalcolumn in the present example) is obtained as shown in FIG. 2A, uponobservation from the viewpoint E2, a second reproduction image B (animage of a star-shaped column in the present example) is obtained asshown in FIG. 2B. That is, two different original images areoverlappingly recorded on the recording medium 10, and mutuallydifferent original images are reproduced according to the observationposition. As a matter of course, the recording medium 10 can be observedfrom a position besides the viewpoint E1 and the viewpoint E2 and insuch a case, only the first reproduction image A is observed, or onlythe second reproduction image B is observed, or both reproduction imagesare observed in a state of being blended at predetermined proportions,in other words, the observation modes vary according to position.

Methods for preparing such a hologram recording medium with thecharacteristic that different original images are reproduced uponobservation from different positions are known as disclosed in theabovementioned Patent Document. However, because as mentioned above, abasic principle of the conventional methods is to set up a plurality ofregions on a hologram recording plane and record a different originalimage on each individual region, there is the problem that thereproduced images are lowered in resolution, and the present inventionproposes a new method that resolves this problem.

FIG. 3 is a flowchart of a basic procedure of the hologram recordingmedium manufacturing method according to the present invention. Thebasic embodiment of the present invention shall now be described in linewith this basic procedure. The flowchart of FIG. 3 illustrates a processfor manufacturing a hologram recording medium by a “computer generatedhologram (CGH)” method and the procedures of steps S1 to S5 are allprocedures that are executed by a computer. A physical hologramrecording medium is formed in a final, pattern forming step of step S6.

First, in an original image preparation step of step S1, a plurality Nof original images are respectively prepared as a set of unit lightsources positioned in a three-dimensional coordinate system. To preparea recording medium that can reproduce the two reproduction images A andB as in the example of FIG. 2's, two original images are prepared instep S1. Thus although in the description that follows, an example whereN=2, that is, an example of a process of preparing two original imagesand manufacturing the hologram recording medium 10 shown in FIG. 2'sshall be described for the sake of convenience, the present inventioncan obviously be applied to cases of N=3 or more.

In a general computer generated hologram method, an optical interferencefringe generation process is simulated on a computer. Thus here, an XYZorthogonal coordinate system is defined as a three-dimensionalcoordinate system in which the optical simulation is performed. FIG. 4'sshow perspective views of an example of two original images defined inthe XYZ coordinate system. FIG. 4A shows a first original image Ia, andFIG. 4B shows a second original image Ib. The first original image Ia ofcylindrical columnar shape, shown in FIG. 4A, is the source of the firstreproduction image A, shown in FIG. 2A, and the second original image Ibof star-shaped columnar shape, shown in FIG. 4B, is the source of thesecond reproduction image B, shown in FIG. 2B.

As shown in FIG. 4's, each of the original images Ia and Ib isconstituted of a plurality of unit light sources that are positioned inthe XYZ coordinate system. Here, it shall be deemed that each unit lightsource is constituted from a point light sources. The respective pointlight sources are indicated in FIG. 4's as black dots (such as P11, P12,P13, P21, P22, and P23). Although examples in which the point lightsources are positioned sparsely are shown in the figures for the sake ofdescription, in actuality, the point light sources are defined at ahigher density in order to prepare original images of higher resolution.

Although in the figures, the first original image Ia is shown in FIG.4A, the second original image Ib is shown in FIG. 4B, and the two aredrawn separately, the two original images Ia and Ib arethree-dimensional images defined in the same XYZ coordinate system, andwith the present example, the two are positioned so as to overlappartially. As shown in the observation modes of FIG. 2's, becausemutually different original images are reproduced from the hologramrecording medium 10 according to the viewpoint position of observation,there is no problem whatsoever even if the plurality of the originalimages prepared in the original image preparation step S1 are positionedso as to partially overlap spatially.

Obviously the respective original images Ia and Ib are images defined ona computer and the actual entities thereof are digital image data. Theoriginal image preparation step of step S1 is thus actually a process ofpreparing original images Ia and Ib, constituted of digital image data,inside a storage unit of a computer. Although in the illustratedexample, both of the original images Ia and Ib are images withthree-dimensional shapes, the original images prepared in the presentinvention do not necessarily have to be three-dimensional images andplanar images (such as character strings positioned in a two-dimensionalplane) may be used as original images as well.

Meanwhile, in a recording plane setting step of step S2, a predeterminedrecording plane 20 is set in the XYZ three-dimensional coordinatesystem, and in a reference light setting step of step S3 that follows, apredetermined reference light R is set in the XYZ three-dimensionalcoordinate system. The recording plane 20 is a plane that corresponds tothe recording surface of the hologram recording medium 10 that is to bethe final product and is normally set up as a flat surface ofrectangular shape. Meanwhile, the reference light R is a light that isused to generate interference fringes on the recording plane 20 byinterference with object light from an original image and is normallyset as a planar wave of predetermined wavelength that is made incidenton the recording plane 20 at a predetermined incidence angle.

An observation region setting step of step S4 is a step of setting aplurality N of observation regions in the XYZ three-dimensionalcoordinate system. Here, an observation region is a region thatindicates a range of viewpoint positions suited for observation of aspecific original image and is a region that is set arbitrarily by apreparer of the hologram recording medium. In the present invention,this observation region serves an extremely important role. Theplurality N of observation regions are set because the plurality N oforiginal images were set in the original image preparation step of stepS1. Put in another way, in step S4, observation regions of a number thatis in accordance with the number of original images prepared in step S1are set. The actual entity and function of the observation regions shallnow be described by way of a specific example.

FIG. 5 is an upper view of an example of the original images Ia and Ib,prepared in the original image preparation step (step S1), the recordingplane 20, set in the recording plane setting step (step S2), thereference light R, set in the reference light setting step (step S3),and observation regions Oa and Ob, set in the observation region settingstep (step S4) in the flowchart of FIG. 3, and shows a state in whichthe XYZ three-dimensional coordinate system, shown in FIG. 4's, isviewed downward along the Z-axis direction. The paper surface of FIG. 5is thus a surface that is parallel to the XY plane. In FIG. 5, the uppersurfaces of the original images Ia and Ib, respectively shown in FIGS.4A and 4B, are shown, and as mentioned above, these original images arepartially overlapped. In the present example, the recording plane 20 isset as a plane perpendicular to the XY plane, and the reference light Ris a planar wave of predetermined wavelength that is made incident onthe recording plane 20 at a predetermined incidence angle.

With the example shown in FIG. 5, because two original images Ia and Ibare prepared, two observation regions Oa and Ob are set in accordancetherewith. The first observation region Oa is a spheroidal (egg-shaped)spatial region, which is indicated by hatching by slanted lines, and thesecond observation region Ob is a spheroidal (egg-shaped) spatialregion, which is indicated by hatching by dots. Here, the firstobservation region Oa is a region set by the preparer of the hologramrecording medium as a region suited for the observation of thereproduction image of the first original image Ia, and the secondobservation region Ob is a region set by the preparer of the hologramrecording medium as a region suited for the observation of thereproduction image of the second original image Ib.

With the illustrated example, because the first observation region Oa isset at a left side in front of the recording plane 20 and the secondobservation region Ob is set at a right side in front of the recordingplane 20, when the hologram recording medium that is prepared by thefollowing procedure is observed from the front left side, the firstoriginal image Ia can be observed, and when the hologram recordingmedium is observed from the front right side, the second original imageIb can be observed.

When the original images Ia and Ib, the recording plane 20, thereference light R, and the observation regions Oa and Ob have been setas shown in FIG. 5, a pattern computation step of step S5 is executed.Here, an interference fringe pattern, which is formed on the recordingplane 20, is computed (interference fringe pattern simulationcomputation) based on the object light emitted from each individual unitlight source that constitutes each original image and the referencelight R.

FIG. 6 is a perspective view of concepts of the computation processperformed in the pattern computation step of step S5. In the illustratedXYZ three-dimensional coordinate system, the first original image Ia andthe second original image Ib are the original images prepared in step S1(as mentioned above, these original images are overlapped partially),the recording plane 20 is the plane set in step S2, the reference lightR is the light set in step S3, and the observation regions Oa and Ob arethe regions set in step S4. In the pattern computation step of step S5,the interference fringe pattern, which is formed on the recording plane20, is determined by computation based on the object light from eachindividual point light source constituting the first original image Ia(in FIG. 6, only light paths of object light components La1 and La2 fromthe point light source P11 are indicated as an example by alternate longand short dash lines), the object light from each individual point lightsource constituting the second original image Ib (in FIG. 6, only lightpaths of object light components Lb1 and Lb2 from the point light sourceP21 are indicated as an example by alternate long and short dash lines),and the reference light R.

In actuality, a plurality of computation points that are alignedvertically and horizontally at predetermined pitches are defined on therecording plane 20 and a computation of determining the amplitudeintensity of light at the position of each individual computation pointis performed. For example, the computation concerning the position of anillustrated computation point C is performed as follows. First, theobject light from each individual light source constituting the firstoriginal image Ia and the object light from each individual light sourceconstituting the second original image Ib that reach the computationpoint C are synthesized to determine a synthetic object light. Theamplitude intensity of an interference wave that is obtained at thecomputation point C by interference of the synthetic object light withthe reference light R is then determined as a density value of theinterference fringe pattern at the computation point C. Such a densityvalue is obtained for each of the plurality of computation pointsdefined on the recording plane 20, and the distribution of these densityvalues becomes the interference fringe pattern to be determined in stepS5.

This computation process of step S5 can generally be defined as aprocess of determining a synthetic object light by synthesizing theobject light components emitted from the respective individual unitlight sources constituting the respective original images anddetermining the interference fringe pattern formed on the recordingplane 20 by the interference of the synthetic object light and thereference light by computation. More specifically, when an object lightcomponent emitted from an individual point light source constituting anoriginal image is expressed by a formula using the complex numberA·exp(−iωt+iφ) (where A is the amplitude, ω is the frequency, t is thetime, φ is the phase, and i is the unit imaginary number), the syntheticobject light for the position of a specific computation point C isdetermined by determining the sum of the above formula for all objectlight components that reach the position, and the interference waveintensity of this synthetic object light and the reference light R atthe position of the computation point C is determined by computation.Because such a computation process in itself is known as a generalmethod for “computer generated holograms,” detailed description thereofshall be omitted here.

A characteristic of the present invention is that in this patterncomputation step of step S5, the observation regions set in step S4 areused to select object light components to be taken into account in thecomputation (hereinafter referred to as “computation-incorporated lightcomponents”). In general terms, when N original images are prepared instep S1 and N observation regions are defined in step S4, computationsthat each takes into account only light components, which, among objectlight components from the unit light sources belonging to an i-th (i=1,2, . . . , N) original image, reach an i-th (i=1, 2, . . . , N)observation region, are performed in the pattern computation step ofstep S5.

With the embodiment described up to now, N=2, the two original images Iaand Ib are prepared in step S1 as shown in FIGS. 4A and 4B, and the twoobservation regions Oa and Ob are set in step S4 as shown in FIG. 5. Thecomputation process of step S5 is thus performed in a state of havingselected computation-incorporated light components as follows. That is,in regard to the object light components from the point light sources(unit light sources) belonging to the first original image Ia, the lightcomponents that reach the first observation region Oa are deemed to bethe computation-incorporated light components and a computation thattakes only the computation-incorporated light components into account isperformed. Likewise, in regard to the object light components from thepoint light sources (unit light sources) belonging to the secondoriginal image Ib, the light components that reach the secondobservation region Ob are deemed to be the computation-incorporatedlight components and a computation that takes only thecomputation-incorporated light components into account is performed.

The object light components La1 and La2, indicated by alternate long andshort dash lines in FIG. 6, are object light components from the pointlight source P11 that belongs to the first original image Ia and becauseboth narrowly meet the condition of reaching the first observationregion Oa (because of passing through a contour line of the observationregion Oa), these are used as computation-incorporated light componentsin the computation in step S5. However, because object light componentsthat pass through the outer side of the region sandwiched by the objectlight components La1 and La2 (object light components that pass to theleft side of La1 or the right side of La2 in the figure) do not reachthe observation region Oa even if these object light components are fromthe point light source P11, these are not computation-incorporated lightcomponents and are ignored in the computation in step S5 (such objectlight components are treated as if not reaching the recording plane 20).

Likewise, the object light components Lb1 and Lb2, indicated byalternate long and short dash lines in FIG. 6, are object lightcomponents from the point light source P21 that belongs to the secondoriginal image Ib and because both narrowly meet the condition ofreaching the second observation region Ob (because of passing through acontour line of the observation region Ob), these are used ascomputation-incorporated light components in the computation in step S5.However, because object light components that pass through the outerside of the region sandwiched by the object light components Lb1 and Lb2(object light components that pass to the left side of Lb1 or the rightside of Lb2 in the figure) do not reach the observation region Ob evenif these object light components are from the point light source P21,these are not computation-incorporated light components and are ignoredin the computation in step S5 (such object light components are treatedas if not reaching the recording plane 20).

Such a computation process shall now be described more specifically. Acase of determining the interference fringe pattern density value for aspecific computation point C, defined on the recording plane 20 in theexample shown in FIG. 6, shall be considered. In this case,conventionally (that is, with the conventional method), the interferencefringe intensity is computed by taking into account the object lightcomponents that propagate toward the computation point C from all pointlight sources constituting the first original image Ia and the objectlight components that propagate toward the computation point C from allpoint light sources constituting the second original image Ib. Thiscomputing method is based on the basic law of physics that “lightemitted from a point light source propagates across the entiresurrounding space.”

Meanwhile, with the present invention, first, for each individual pointlight source, a task of judging whether or not an object light componentemitted from the point light source and propagating toward thecomputation point C is a computation-incorporated light component isperformed in reference to an observation region defined in step S4. Withthe example shown in FIG. 6, the object light component La3, which isemitted from the point light source P11 and propagates toward thecomputation point C, is not a computation-incorporated light component.This is because, even if the light path of the object light componentLa3, indicated by the alternate long and short dash line in the figure,is extended downward, it does not reach the observation region Oa(whether or not the observation region Ob is reached is an irrelevantmatter). Because the point light source P11 is a point light sourceconstituting the first original image Ia, any light component, among theobject light components emitted from the point light source P11, thatdoes not reach the first observation region Oa is not taken into accountin the computation. The second observation region Ob is not involved injudgments concerning the object light components from the point lightsource P11.

In determining the interference fringe intensity for the computationpoint C, the same judgment process is performed on each of all pointlight sources constituting the first original image Ia and furthermoreon each of all point light sources constituting the second originalimage Ib. What is important here is that whereas in judging whether ornot an object light component, emitted from a point light sourceconstituting the first original image Ia, is a computation-incorporatedlight component, the criteria of whether or not the object lightcomponent reaches the first observation region Oa is used, in judgingwhether or not an object light component, emitted from a point lightsource constituting the second original image Ib, is acomputation-incorporated light component, the criteria of whether or notthe object light component reaches the second observation region Ob isused.

When by such a judgment task, the judging of whether an object lightcomponent is a computation-incorporated light component has beencompleted for all object light components propagating toward a specificcomputation point C from all point light sources constituting theoriginal images Ia and Ib, the interference fringe intensity at theposition of the specific computation point C can be computed. That is, acomputation that takes into account only the light components that havebeen judged to be the computation-incorporated light components fromamong all of the object light components propagating toward the positionof the computation point C is performed. By executing such a computationfor each individual computation point C on the recording plane 20, adistribution of the interference fringe intensity values on therecording plane 20 is obtained in the form of an interference fringepattern.

When the interference fringe generation simulation is performed by sucha method, as shown in FIG. 7, in regard to object light components frompoint light sources P14, P15, and P16 and other point light sources ofthe first original image Ia, computations that take into account onlythe light components that propagate toward the first observation regionOa is performed as indicated by the slanted-line hatching. Also, asshown in FIG. 8, in regard to object light components from point lightsources P24, P25, and P26 and other point light sources of the secondoriginal image Ib, computations that take into account only the lightcomponents that propagate toward the second observation region Ob isperformed as indicated by the dot hatching. As a result, when, as in theobservation modes shown in FIG. 2's, the hologram recording medium 10 isobserved from the left viewpoint E1 (a position inside the firstobservation region Oa), the first reproduction image A (the reproductionimage of the first original image Ia) is observed, and when the hologramrecording medium 10 is observed from the right viewpoint E2 (a positioninside the second observation region Ob), the second reproduction imageB (the reproduction image of the second original image Ib) is observed.

In theory, when after recording the interference fringe pattern on therecording plane 20 upon setting the respective object light componentsand the respective reference light components R to the samemonochromatic light (that is, light of the same, single wavelength) inperforming the above-described interference fringe generationsimulation, observation is performed upon illuminating the recordingplane 20 with a reproduction illumination light of the same wavelengthas the reference light R from the same direction, the first reproductionimage A is observed only when the viewpoint is set inside the firstobservation region Oa and second reproduction image B is observed onlywhen the viewpoint is set inside the second observation region Ob.However, in actuality, image reproduction of a hologram recording mediumthat is used in a cash voucher or credit card is generally carried outunder an indoor illumination environment containing various wavelengthcomponents (an illumination environment close to white light), and thewavelength and illumination direction of the reproduction illuminationlight do not match those of the reference light R.

Thus, in actuality, the first reproduction image A is not necessarilyobserved only from inside the first observation region Oa, and thesecond reproduction image B is not necessarily observed only from insidethe second observation region Ob. However, because a relationship, wherethe first reproduction image A is mainly reproduced upon observationfrom a vicinity of the first reproduction region Oa and the secondreproduction image B is mainly reproduced upon observation from avicinity of the second reproduction region Ob, is maintained, the objectthat different original images are reproduced upon observation fromdifferent positions can be achieved.

The pattern forming step of step S6 that is indicated as the last stepin the flowchart shown in FIG. 3 is a step of forming the interferencefringe pattern, determined in step S5, on a physical medium. In thisstep, any method may be employed as long as the density pattern ofinterference fringes can be formed in some way on a physical medium.Because various methods are already known as such a method, a detaileddescription shall be omitted here, and generally, a method of convertingthe interference fringe pattern, obtained in the pattern computationstep of step S5, into a binary image pattern and forming the binaryimage pattern on a physical medium is employed widely. For example, aplanar medium, constituted of the two colors of black and white, athree-dimensional structure medium, constituted of the two types ofportions of recessed portions and protruding portions, etc., aregenerally used. Because the interference fringe pattern is an extremelyfine pattern that gives rise to optical interference, in terms ofpractical use, a method, in which the fine pattern that is to be formedis provided to an electron beam printer and physical interferencefringes are formed by scanning an electron beam across a medium isemployed in many cases.

Section 2. Other Embodiments Related to Observation Regions

A basic embodiment of the present invention was described above inSection 1. In summary, the basic philosophy of the present invention isto perform, in the pattern computation step, computations that take intoaccount only the object light components, among the object lightcomponents from each unit light source, that propagate toward a uniqueobservation region, which is set in accordance with the original imageto which the unit light source belongs. With the example shown in FIG.6, in regard to the object light components from the unit light sourcesbelonging to the first original image Ia, a computation, which takesinto account only the object light components, which, among all objectlight components, propagate toward the first observation region Oa, isperformed, and in regard to the object light components from the unitlight sources belonging to the second original image Ib, a computation,which takes into account only the object light components, which, amongall object light components, propagate toward the second observationregion Ob, is performed. An observation region that is set in thepresent invention is thus a region that functions as a basis forselecting the computation-incorporated light components to be taken intoaccount in computation from among the object light components from theunit light sources, and any type of region may be set as an observationregion as long as such a function can be served. Here, other embodimentsrelated to the setting of the observation region shall be described.

(1) Shape of the Observation Region

Although an example, in which observation regions Oa and Ob withspheroidal (egg-like) shapes are set, is shown in FIGS. 5 and 6, theshape of each observation region set in step S4 is not restricted to aspecific shape, and observation regions of arbitrary shapes can be setaccording to the intentions of the preparer. The sizes of theobservation regions can also be set arbitrarily. Because as mentionedabove, an observation region is a region used for selection of whetheror not each object light component is to be a computation-incorporatedlight component, the shape and size of the observation region may bearbitrary. An observation region also does not necessarily have to be aregion constituted of a three-dimensional body and may be a regionconstituted of a plane or a curved surface. That is, because it sufficesthat judgment, of whether or not the light components emitted from therespective unit light sources constituting an original image can reachan observation region, is possible, various observation regions can beset as a plane, a curved surface, or a three-dimensional body in athree-dimensional coordinate system.

However, because the shape of an observation region is a matter thatinfluences the observation modes of the hologram recording medium thatis prepared in the final stage, it is preferably set as a region with acomparatively simple shape for practical use.

(2) Position of the Observation Region

The position of each observation region is also an important matter thatinfluences the observation modes of the hologram recording medium thatis prepared in the final stage. An example, where the observationregions Oa and Ob are set in front of the recording plane 20 as tworegions that are separated to the left and right, is shown in FIGS. 5and 6. When two observation regions are thus positioned to the left andright, it becomes possible to prepare a hologram recording medium withthe characteristic that the first reproduction image A (original imageIa) is observed upon observation from a generally left side, and thesecond reproduction image B (original image Ib) is observed uponobservation from a generally right side. Meanwhile, when the twoobservation regions are positioned at upper and lower sides, a hologramrecording medium, with the characteristic that the first reproductionimage A is observed upon observation from the upper side, and the secondreproduction image B is observed upon observation from the lower side,can be prepared.

Also, although an example where the observation regions are set at theopposite side of the recording plane 20 with respect to the originalimages is shown in FIGS. 5 and 6, the observation regions may be set atthe same side as the original images. For example, in FIG. 6, therecording plane 20 is positioned in front of the original images Ia andIb and the observation regions Oa and Ob are set in front of therecording plane 20. However, the observation regions do not necessarilyhave to be positioned at the front side of the recording plane 20 andmay be positioned at the other side of the recording plane. Observationregions Oa′ and Ob′, indicated by broken lines in FIG. 6, are regions ofan example in which the observation regions are set at the other side(the same side as the original images) of the recording plane 20. Thelight components, which, among the object light components emitted fromthe point light sources constituting the first original image Ia, reachthe observation region Oa′ (that is, the light components that propagatetoward the recording plane 20 upon passing through the observationregion Oa′), become the computation-incorporated light components, andlikewise, the light components, which, among the object light componentsemitted from the point light sources constituting the second originalimage Ib, reach the observation region Ob′ (that is, the lightcomponents that propagate toward the recording plane 20 upon passingthrough the observation region Ob′), become the computation-incorporatedlight components in this case as well.

When such settings are made, the observation regions Oa′ and Ob′ losethe significance as regions for placing viewpoints and become regionsthat serve as a basis for judging computation-incorporated lightcomponents. In FIG. 6, because the light paths of the object lightcomponents from the point P11 are indicated by alternate long and shortdash lines, in regard to the object light components from point P11, thelight components that pass within the observation region Oa′ reach theinterior of the observation region Oa. However, in regard to objectlight components from other points, light components that pass withinthe observation region Oa′ do not necessarily reach the interior of theobservation region Oa. Setting of the observation region Oa′ is thus notequivalent to setting the observation region Oa. Actually, with theexample shown in FIG. 6, although when recording is performed uponsetting the observation region Oa′, a portion of the original image Iaappears chipped upon observation from a viewpoint within the observationregion Oa, in the case of an application with which there is no problemwith such an observation mode, the observation region Oa′ can be set inplace of Oa.

(3) Interrelationship Among the Plurality of Observation Regions

The two observation regions Oa and Ob shown in FIGS. 5 and 6 are regionsthat are spatially exclusive with respect to each other and there is nospatial overlapping between the two. A basic function of the hologramrecording medium according to the present invention is the function thatdifferent original images are reproduced when observed from differentpositions, and in realizing such a function, it is preferable, in theprocess of setting the plurality N of observation regions in theobservation region setting step, to set the regions to be spatiallyexclusive regions with respect to each other.

However, as mentioned above, in an actual environment in which thehologram recording medium is reproduced, because the wavelength andillumination direction of the reproduction illumination light do notmatch those of the reference light R, even if the two observationregions Oa and Ob are set as regions that are spatially exclusive withrespect to each other as shown in FIGS. 5 and 6, a phenomenon that anunintended reproduction image is observed from an unintended positioncan occur. For example, the first reproduction image A (first originalimage Ia) may be observed even upon observation from within a regionbesides the observation region Oa, and the second reproduction image B(second original image Ib) may be observed even upon observation fromwithin a region besides the observation region Ob.

In consideration of the above, for practical use, the plurality N ofobservation regions do not necessarily have to be set as regions thatare spatially exclusive with respect to each other. In actuality, aportion or all of the plurality N of observation regions may be set asregions that partially overlap spatially with another observationregion. A hologram recording medium that is prepared using such settingsof partially overlapping regions becomes a recording medium with whichthe occurrence of a phenomenon that a plurality of original images areobserved simultaneously upon observation from a specific position ispresumed in advance.

For example, when with the example shown in FIGS. 5 and 6, a right sideportion of the first observation region Oa and a left side portion ofthe second observation region Ob are overlapped spatially, even when areproduction illumination light that is exactly the same as thereference light is used, a point, at which both the reproduction imagesA and B are observed overlappingly, becomes present at a positionintermediate the viewpoint E1 shown in FIG. 2A and the viewpoint E2shown in FIG. 2B. Thus, as the viewpoint is moved gradually from theleft side to the right side, the observed reproduction image changesgradually from the first reproduction image A to the second reproductionimage B. To realize such an observation mode, two observation regionsare set so as to overlap with each other partially.

(4) Setting the Same Observation Region

According to the basic principles of the present invention, when aplurality N of original images are prepared in the original imagepreparation step (step S1), the same plurality N of observation regionsare set in the observation region setting step (step S4), and in thepattern computation step (step S5), computations that each takes intoaccount only the light components, which, among object light componentsfrom the unit light sources belonging to an i-th (i=1, 2, . . . , N)original image, reach the i-th (i=1, 2, . . . , N) observation region,are performed. For example, when N=3, three original images and threeobservation regions are prepared, and in regard to object lightcomponents from the first original image, a computation that takes intoaccount only the light components that reach the first observationregion is performed, in regard to object light components from thesecond original image, a computation that takes into account only thelight components that reach the second observation region is performed,and in regard to object light components from the third original image,a computation that takes into account only the light components thatreach the third observation region is performed.

Here, although as has been described in (3) above, a portion or all ofthe N observation regions may be set as regions that partially overlapspatially with another observation region, a portion or all of the Nobservation regions may also be set as regions that spatially matchanother observation region completely. For example, when three originalimages and three observation regions are prepared as mentioned above,the region Oa shown in FIGS. 5 and 6 may be set as the first observationregion and the second observation region, and the region Ob may be setas the third observation region. In this case, the first observationregion and the second observation region are set as regions that arecompletely the same (obviously, a portion of the observation region Oaand a portion of the propagation Ob may be overlapped spatially).

By such settings, a hologram recording medium can be prepared with whichthe first reproduction image and the second reproduction image can beobserved upon observation from the first viewpoint E1 and the thirdreproduction image can be observed upon observation from the secondviewpoint E2. With such a hologram recording medium, because the firstobservation region and the second observation region are set to the sameregion, the first original image and the second original image arerecorded under the same conditions and, as a result, the observationmodes of the first reproduction image and the second reproduction imageare the same. That is, if upon observation from a certain viewpoint, thefirst reproduction image can be observed, the second reproduction imagecan also be observed at the same time. However, because the thirdobservation region is set as a separate region, the third original imageis recorded under separate conditions and the observation mode of thethird reproduction image is different.

That “different original images are reproduced upon observation fromdifferent positions” in the present invention does not mean that “for aplurality N of original images, just one of the original images can beobserved upon observation from a certain specific position” but meansthat “the combination of observable original images, among a plurality Nof original images, changes when the observation position is changed.”

Section 3. Embodiment of Applying Restrictions by Plate-Like Spaces

Methods, of performing computation upon applying some form ofrestriction on the spreading of an object light from unit light sourcesconstituting an original image in preparing a computer generatedhologram, are known. For example, Japanese Patent Laid-open PublicationsNo. H11-24539A and No. H11-202741A disclose methods of computinginterference fringe intensities upon restricting the spreading of anobject light from a point light source within a space defined by apredetermined angle of spread. Such “computation performed uponrestricting the spread angle of the object light” is equivalent to a“computation performed by taking into account only light componentsinside a predetermined spread angle,” and from this standpoint, thetechnical philosophy of the “computation that takes into account only aportion of the object light” has already been disclosed in theabovementioned patent publications. For example, with the example shownin FIG. 6, “computation is performed by taking into account only thelight components, which, among the object light components emitted fromthe point light source P11, reach the observation region Oa,” and such amanner of handling is equivalent to “performing computation uponrestricting the spread angle of the object light emitted from the pointlight source P11 within a subulate region sandwiched by the alternatelong an short dash lines La1 and La2.”

However, the purpose of “restricting the spread angle of the objectlight” in the method disclosed in these publications is to suppressluminance non-uniformity or to lighten the computational load and is notto enable reproduction of different original images upon observationfrom different positions as in the present invention. As a matter ofcourse, the important characteristic of the present invention of settinga unique observation region (setting an object light spread angle)according to each individual original image is not disclosed whatsoeverin these publications.

Although the actions and effects of the “restriction of the spread angleof object light” in the present invention and the actions and effects ofthe “restriction of the spread angle of object light” in theabovementioned known examples thus differ completely in principle,because these share the point that the spread angle of object light isrestricted in some form in computing the interference fringeintensities, the two can be used in combination. Put in another way, inputting the present invention into practice, the “restriction of thespread angle of object light” of the abovementioned known example can beapplied overlappingly. An example of such an embodiment shall now bedescribed.

FIG. 9 is a sectional view of concepts of performing interference fringepattern computation upon partitioning a three-dimensional space, definedby the XYZ coordinate system, into a plurality of plate-like spaces andapplying a weighting condition that the recording plane is reached onlyby propagation through a single plate-like space as a condition forobject light components, emitted from the respective point lightsources, to be computation-incorporated light components. That is, withthe present embodiment, in the selection of object light components,emitted from the respective point light sources, ascomputation-incorporated light components, not only the condition of“reaching a specific observation region” (the above-described conditionthat is a characteristic of the present invention), but the weightingcondition of “propagating through one and only one plate-like spaceuntil at least the recording plane is reached” is also applied. From thestandpoint of “restricting of the spread angle of object light,” theabove means that a spread angle restriction that meets both thecondition of a “spread angle enabling reaching of a specific observationregion” and the condition of a “spread angle enabling the reaching ofthe recording plane through just one plate-like space” is applied.

With the illustrated example in FIG. 9, the three dimensional space ispartitioned into a plurality of plate-like spaces G1 to G7 by sevenslicing planes H1 to H7, indicated by broken lines. Here, each of theslicing planes H1 to H7 is a plane parallel to the XY plane. Forexample, the plate-like space G1 is a space sandwiched between theslicing planes H1 and H2, the plate-like space G2 is a space sandwichedbetween the slicing planes H2 and H3, and the plate-like space G7 is aspace sandwiched between the slicing plane H7 and the XY plane.

In the figure, Ia is a first original image of cylindrical columnarshape, and P17, P18, and P19 are examples of point light sources thatconstitute the first original image Ia. A plane 20 at the right side ofthe figure is a recording plane defined in the XYZ coordinate system,and an interference fringe intensity is computed for each of a pluralityof computation points positioned on the recording plane 20 (theillustration of the reference light R is omitted).

As mentioned above, with the present invention, computation that takesinto account only the light components, which, among the object lightcomponents emitted from the point light sources constituting the firstoriginal image Ia, reach the first observation region (not shown in FIG.9), is performed. With the embodiment described here, as a furtherweighting condition, only light components, which, among the objectlight components emitted from a specific point light source, reach therecording plane 20 upon propagating through only the plate-like space towhich the point light source belongs, are handled as thecomputation-incorporated light components. Put in another way, even if alight component reaches the first observation region, the lightcomponent does not become a computation-incorporated light component ifit reaches the recording plane 20 via a plurality of the plate-likespaces.

For example, because the illustrated points P17, P18, and P19 are pointlight sources positioned inside the first plate-like space G1, only theobject light components from the point light sources P17, P18, and P19that satisfy both a first condition of reaching a predeterminedobservation region that has been set in advance (the observation regiondefined in correspondence to the first original image Ia; not shown inFIG. 9) and a second condition of reaching the recording plane 20 uponpropagating through only the first plate-like space G1 are taken intoaccount as subjects of computation.

Here, if, of the recording plane 20, the region sandwiched between theslicing planes H1 and H2 is referred to as a unit recording region U1 asshown in the figure, then among the object light components from thepoint light sources P17, P18, and P19 positioned in the plate-like spaceG1, the object light components, which are to be taken into account inthe computation of the interference fringe pattern formed on therecording plane 20, are restricted to only the light components that“reach the predetermined observation region set in advance” and “reachthe unit recording region U1.” That is, although the object lightcomponents from the point light sources P17, P18, and P19 are emitted tothe entire space inside the XYZ coordinate system, in the embodimentdescribed here, of these object light components that are emitted to theentire space, “the light components that do not reach the predeterminedobservation region set in advance” and “the light components that do notreach the unit recording region U1 (the light components that propagateout of the plate-like space G1 before reaching the recording plane 20)”are not taken into account whatsoever in the computation of theinterference fringe pattern.

Put in another way, the object light components from the point lightsources P17, P18, and P19 are used only in the interference fringeintensity computation for a computation point positioned inside theillustrated unit recording region U1 (a strip-like region that extendsin the direction perpendicular to the paper surface) and are notinvolved whatsoever in computations concerning computation pointspositioned at other positions. Obviously, the object light componentsfrom the point light sources P17, P18, and P19 are not necessarily usedin the interference fringe intensity computations for all computationpoints inside the unit recording region U1 but are used only in theinterference intensity computations of computation points, each of whichsatisfies being “a computation point that is within the unit recordingregion U1” and being “a computation point, for which light propagatingthereto reaches the interior of the predetermined observation regionthat has been set in advance.”

Thus, in general, the characteristic of the embodiment described here isthat in the pattern computation step, a three-dimensional space ispartitioned into a plurality M of plate-like spaces by slicing by aplurality of mutually parallel planes and computations that each takesinto account only light components, which, among object light componentsfrom unit light sources, inside a j-th (j=1, 2, . . . M) plate-likespace and belonging to an i-th (i=1, 2, . . . , N) original image, reachan i-th (i=1, 2, . . . , N) observation region and reach the recordingplane only through the j-th (j=1, 2, . . . , M) plate-like space, areperformed.

By thus applying the condition (the condition of reaching apredetermined observation region) that is a characteristic of thepresent invention and a condition based on a conventionally known method(for example, the condition of passing through only one plate-like spaceuntil the recording plane is reached as described above) in an ANDcondition to perform the interference fringe intensity computation, asynergistic effect of the actions and effects unique to the presentinvention (the making of different original images be reproduced uponobservation from different positions) and the actions and effects uniqueto the conventionally known method (suppression of luminancenon-uniformity and lightening of the computation load) can be obtained.

Thus, although an essential basic concept of the present invention is to“perform computations by taking into account only light components,which, among object light components from a unit light source, reach aspecific observation region,” this does not mean that “light components,which, among object light components from a unit light source, reach aspecific observation region are always taken into account in thecomputations.” When as in the example described above, a weightingcondition is added as a condition for selecting object light componentsas computation-incorporated light components, obviously “a lightcomponent, which, among object light components from a unit lightsource, reaches a specific observation region” does not become acomputation-incorporated light component unless the weighting conditionis satisfied. That is, with the present invention, to “perform acomputation by taking into account only light components, which, amongobject light components from a unit light source, reach a specificobservation region” can be put in another way as “not taking intoaccount light components, which, among object light components from aunit light source, do not reach the specific observation region.”

Section 4. Embodiment Using Segment Light Sources

With the embodiments described up to now, examples using point lightsources as the unit light sources that constitute an original image havebeen described. However, in putting the present invention into practice,the individual unit light sources that constitute an original image donot necessarily have to be point light sources. For example, by defininga segment light source as a locus of moving a point light source P,shown in FIG. 10A, by just d/2 in each of the upward and downwarddirections along the Z-axis, a segment light source PP with a length dcan be defined as shown in FIG. 10B. Each of the original imagesprepared in step S1 of the present invention may be constituted of acollection of such segment light sources PP.

For example, although the original image Ia shown in FIG. 4A and theoriginal image Ib shown in FIG. 4B are both constituted of collectionsof point light sources, by defining segment light sources as the loci ofmoving the respective individual point light sources upward and downwardalong the Z-axis by just d/2, the respective original images Ia and Ibcan be handled as collections of segment light sources of length d.

In general, a point light source is a light source that emits objectlight that is constituted of a spherical wave and an object light from apoint light source spreads radially with the position of the point lightsource as the center. Meanwhile, because light from a segment lightsource is not a spherical wave, a segment light source must be handledin a slightly different manner from a point light source.

One method of handling a segment light source is to handle it as a lightsource that is formed by aligning a plurality of point light sourcesalong a segment of predetermined length, that is, as a light sourceconstituted of a set of point light sources. When a segment light sourceis thus handled as a collection of point light sources, the object lightcan be defined as a synthetic wave of spherical waves emitted radiallyfrom the respective point light sources. For example, the segment lightsource PP shown in, FIG. 10B can be handled as a collection of aplurality of point light sources from a point light source positioned ata lower end of the segment to a point light source positioned at anupper end of the segment, and the object light can be defined as asynthetic wave of spherical waves emitted radially respectively from theindividual point light sources.

Another method of handling a segment light source is a method that is inaccordance with a line light source. The wavefront of an object lightemitted from a theoretical line light source (a line light source ofinfinite length) is a side surface of a cylindrical column having theposition of the line light source as central axis. For example, in thecase of a line light source extending along the Z-axis, the wavefront isa side surface of a cylindrical column having the Z-axis as the centralaxis, all object light components propagate in directions orthogonal tothe Z-axis, and there are no object light components that propagate in adirection along the Z-axis. Although a segment light source is actuallya light source of finite length, it can be handled in a manner that isin accordance with a line light source. In this case, the wavefront ofan object light emitted from the segment light source is a side surfaceof a cylindrical column having the position of the segment light sourceas the central axis and there are no object light components thatpropagate in a direction along the segment light source. An example ofsuch handling of a segment light source is disclosed in Japanese PatentLaid-open Publication No. 2001-013858A.

As an example shown in FIG. 11, a segment light source QQ, constitutedof a segment with a length d (a segment joining a lower end point Q andan upper end point Q′), shall now be considered with the lower end pointQ being defined at the position of the origin of the XYZthree-dimensional coordinate system and the upper end point Q′ beingdefined at a position along the Z-axis that is separated from the lowerend point Q by just the distance d. When this segment light source QQ ishandled in a manner that is in accordance with a line light source, anobject light emitted from an arbitrary position on the segment lightsource QQ is light that spreads radially from the Z-axis as the centerand along a plane that passes through the arbitrary position and isparallel to the XY plane. To describe with a specific example, an objectlight emitted from the upper end point Q′ in the figure is light thatpropagates radially along a plane, expressed by the formula: Z=d, so asto move away from the upper end point Q′, and the directions ofpropagation of all object light components are orthogonal to the Z-axis.Put in another way, the wavefront of the object light emitted from thesegment light source QQ is a side surface of a cylindrical column ofheight d having the Z-axis as the center axis as illustrated.

When the segment light source QQ is handled in such a manner that is inaccordance with a line light source, results close to those of the“embodiment of applying restrictions by plate-like spaces,” which wasdescribed in Section 3, are obtained. Although an example in which thethree-dimensional space, formed by the XYZ coordinate system, ispartitioned by slicing planes H1 to H7 to form the plurality ofplate-like spaces G1 to G7 is shown in FIG. 9, a case where the intervalof each of the slicing planes H1 to H7 is set to d and the individualpoint light sources are replaced by segment light sources of length dthat respectively fit inside the corresponding plate-like spaces shallbe considered here.

For example, FIG. 12 shows an example where the point light sources P17,P18, and P19, shown in FIG. 9, are respectively replaced by segmentlight sources PP17, PP18, and PP19. Each of the segment light sourcesPP17, PP18, and PP19 has a length d and fits exactly inside theplate-like space G1. Because when each segment light source is handledin a manner that is in accordance with a line light source, the objectlight propagates only in horizontal directions (directions parallel tothe XY plane) in FIG. 12, the object light components from the segmentlight sources PP17, PP18, and PP19 reach only points within the unitrecording region U1 by propagating only through the interior of theplate-like space G1.

Obviously in this case, not all of the object light components from thesegment light sources PP17, PP18, and PP19 are selected ascomputation-incorporated light components for all computation pointsinside the unit recording region U1. For selection as acomputation-incorporated light component, the basic condition that thespecific observation region, which is not illustrated, is reached mustobviously be satisfied. However, if the illustrated segment lightsources PP17, PP18, and PP19 are handled in a manner that is inaccordance with line light sources (that is, if the light sources arehandled in a manner such that the wavefront of the object light is aside surface of a cylindrical column such as shown in FIG. 11), becausethe object light components from the segment light sources PP17, PP18,and PP19 necessarily reach the interior of the unit recording region U1by propagating only through the interior of the plate-like space G1, theweighting condition, described in Section 3 is always satisfied.

Although an example of using segment light sources in place of pointlight sources as the unit light sources constituting an original imagewas described above, besides this, plane light sources may also be usedas the unit light sources. For example, in a case of using an originalimage that is defined as a collection of polygons, each individualpolygon may be handled as a plane light source.

Section 5. Embodiments in Which the Reference Light is Not Set

In the pattern computation step in the embodiments described up to now,the interference fringe pattern formed on the recording plane 20 iscomputed based on the object light, emitted from each individual unitlight source constituting an original image, and the reference light.However, in recording the information of the original image as ahologram on the recording plane 20 using the “computer generatedhologram” method, the information do not necessarily have to be recordedin the form of an interference fringe pattern. Put in another way, thereference light does not have to be set necessarily.

Generally, in an optical hologram recording method using a silver halidefilm, because an original image must be recorded as an interferencefringe pattern on the silver halide film that is to be the recordingplane, a reference light must be prepared in addition to the objectlight and these two must be made to interfere with each other. However,theoretically, as long as information on the amplitude and phase(complex amplitude) of a synthetic wave, obtained by synthesizing allobject light components arriving from an original image, are recorded onthe recording plane, the original image can be reproduced. Because byusing the “computer generated hologram” method, a complex amplitudepattern that is formed on the recording plane 20 can be determined fromthe object light by performing a computation based on the amplitude andphase of the arriving light and without setting a reference light, ahologram recording medium can be prepared by forming this complexamplitude pattern in some form on a physical medium.

That is, with the embodiment described in this Section 5, although thesame procedures as those of the embodiments described above areperformed in the “original image preparation step” of step S1, in the“recording plane setting step” of step S2 and in the “observation regionsetting step” of step S4 in the flowchart of FIG. 3, the “referencelight setting step” of step S3 is unnecessary.

Also, in the “pattern computation step” of step S5, instead of computingan interference fringe pattern, a complex amplitude pattern, which isformed on the recording plane 20 by synthesis of the object lightcomponents emitted from the individual unit light sources constitutingthe respective original images, is computed. Obviously, the point that,in handling the object light components from the respective unit lightsources, computation is performed upon deeming that only lightcomponents which reach a certain observation region are taken intoaccount, is exactly the same as that of the embodiments described thusfar.

Specifically, the complex amplitude pattern is computed as follows. Thatis, an object light emitted from an individual point light sourceconstituting an original image is expressed by a formula using thecomplex number: A·exp(−iωt+iφ) (where A is the amplitude, ω is thefrequency, t is the time, φ is the phase, and i is the unit imaginarynumber), and for the position of a specific computation point C, the sumof these formulae for all computation-incorporated light components thatreach the position is determined. The formula expressing this sum isalso expressed using the complex number: A·exp(−iωt+iφ) and indicatescomplex amplitude information (amplitude information and phaseinformation). A complex amplitude pattern (a distribution pattern ofamplitude values and phase values) can thus be obtained on the recordingplane 20.

The equation using “A·exp(−iωt+iφ)” contains the time t as a parameterand the amplitude and phase are quantities that vary with time. Thus, inactuality, a specific sampling time point is set (t is provided with anarbitrary value (such as 0)) and the complex amplitude pattern on therecording plane 20 at this sampling time point is determined.Specifically, a process, of defining a plurality of computation pointsdiscretely on the recording plane 20, determining the amplitude and thephase of the synthetic object light at the predetermined sampling timepoint at each computation point position, and thereby determining thecomplex amplitude pattern as a discrete distribution of amplitudes andphases, is performed.

Meanwhile, in the “pattern forming step” of step S6, the complexamplitude pattern must be formed in place of an interference fringepattern (contrasting density pattern) on a physical medium. As mentionedabove, because the complex amplitude pattern is a pattern havinginformation on both amplitudes and phases, not only an amplitude but aphase must also be recorded on a predetermined position of a physicalmedium. In addition, in order for a correct hologram reproduction imageto be obtained upon illumination of a reproduction illumination lightonto the medium, optical modulation that is in accordance with theamplitude and phase recorded at each individual position must beperformed on the reproduction illumination light made incident on themedium.

As one method of forming such a complex amplitude pattern on a physicalmedium, the inventor of the present Application proposes a method ofusing a plurality of cells with a three-dimensional structure. Insummary with this method, a cell, constituted of a three-dimensionalstructure, is positioned at each individual computation point positionof the recording plane 20 and information on the amplitude and the phaseof the computation point position corresponding to the cell is recordedin the three-dimensional structure of each individual cell. A specificthree-dimensional structure of an individual cell is disclosed, forexample, in U.S. Pat. Nos. 6,618,190 and 6,934,074, etc., and detaileddescription thereof shall be omitted here.

Section 6. Manufacturing Device According to the Present Invention

Lastly, basic arrangements of hologram recording medium manufacturingdevices according to the present invention shall be described withreference to the block diagrams of FIGS. 13 and 14. The manufacturingdevice shown in FIG. 13 is a device for executing steps S1 to S6 shownin the flowchart of FIG. 3 and has a function of manufacturing ahologram recording medium with an arrangement such that differentoriginal images are reproduced upon observation from differentpositions.

In FIG. 13, an original image storage unit 110 is a component forstoring the information of the original images prepared in the “originalimage preparation step” of step S1 and has a function of storing theplurality N of original images respectively as data indicating sets ofunit light sources positioned in the XYZ three-dimensional coordinatesystem.

Also, a recording plane setting unit 120 is a component for executingthe “recording plane setting step” of step S2 and has a function ofperforming a process of setting the predetermined recording plane 20 inthe XYZ three-dimensional coordinate system. A reference light settingunit 130 is a component for executing the “reference light setting step”of step S3 and has a function of performing a process of setting thepredetermined reference light R in the XYZ three-dimensional coordinatesystem. Meanwhile, an observation region setting unit 140 is a componentfor performing the “observation region setting step” of step S4 and hasa function of setting the plurality N of observation regions in the XYZthree-dimensional coordinate system. In actuality, the recording planesetting unit 120, the reference light setting unit 130, and theobservation region setting unit 140 can be realized by input devices anddata storage devices (any of various memories, hard disk devices, etc.)for a computer and dedicated programs for the setting processes.

A pattern computation unit 150 is a component for executing the “patterncomputation step” of step S5 and performs a process of computing aninterference fringe pattern formed on the recording plane 20 based onthe object light components, emitted from the individual unit lightsources constituting the respective original images, and the referencelight R. Here, as was described above, computations that each takes intoaccount only light components, which, among object light components fromeach unit light source belonging to an i-th (i=1, 2, . . . , N) originalimage, reach an i-th (i=1, 2, . . . , N) observation region, areperformed. In actuality, the pattern computation unit 150 can berealized by a dedicated program installed in a computer.

Furthermore, a pattern forming unit 160 is a component having a functionof forming the interference fringe pattern, determined by the patterncomputation unit 150, on a physical medium. Specifically, the patternforming unit 160 can be arranged from, for example, an electron beamprinter and computer that controls it.

Meanwhile, the manufacturing device shown in FIG. 14 is a manufacturingdevice for carrying out the embodiment of not setting a reference lightthat was described in Section 5. The original image storage unit 110,the recording plane setting unit 120, and the observation region settingunit 140 are exactly the same as the respective components shown in FIG.13. However, the device shown in FIG. 14 does not have the referencelight setting unit 130. A pattern computation unit 155 performs aprocess of computing a complex amplitude pattern (distribution patternof amplitudes and phases), which is formed on the recording plane 20 bysynthesizing the object light components (computation-incorporated lightcomponents) emitted from the individual unit light sources constitutingthe respective original images, as was described in Section 5. A patternforming unit 165 performs a process of forming the complex amplitudepattern, determined by the pattern computation unit 155, on a physicalmedium as was described in Section 5.

For practical use, the components in FIG. 13 that are surrounded byalternate long and short dash lines (the original image storage unit110, the recording plane setting unit 120, the reference light settingunit 130, the observation region setting unit 140, and the patterncomputation unit 150) can be realized by installing dedicated processingprograms in one or a plurality of general-purpose computers 170.Likewise, the components in FIG. 14 that are surrounded by alternatelong and short dash lines (the original image storage unit 110, therecording plane setting unit 120, the observation region setting unit140 and the pattern computation unit 155) can also be realized byinstalling dedicated processing programs in one or a plurality ofgeneral-purpose computers 175.

1. A method for manufacturing a hologram recording medium that has anarrangement by which different original images are reproduced whenobserved from different positions, the hologram recording mediummanufacturing method comprising: an original image preparation step ofpreparing a plurality N of original images, each as a set of unit lightsources positioned in a three-dimensional coordinate system; a recordingplane setting step of setting a predetermined recording plane in thethree-dimensional coordinate system; an observation region setting stepof setting a plurality N of observation regions in the three-dimensionalcoordinate system; a pattern computation step of computing a complexamplitude pattern formed on the recording plane by synthesis of objectlight components emitted from the individual unit light sourcesconstituting the respective original images; and a pattern forming stepof forming the complex amplitude pattern on a physical medium; whereinone-to-one correspondences are made between said plurality N of originalimages and said plurality N of observation regions, respectively; andwherein in the pattern computation step, a plurality of computationpoints being defined on said recording plane, at each of computationpoint positions, a synthetic object light is obtained by synthesizingobject lights, emitted from the individual unit light sourcesconstituting said plurality N of original images, which reach said eachof computation point positions to compute a complex amplitude of saidsynthetic object light, wherein computations that each takes intoaccount only light components, which, among the object light componentsfrom the unit light sources belonging to an i-th (i=1, 2, . . . , N)original image, reach an i-th (i=1, 2, . . . , N) observation regioncorresponding to said i-th original image, are performed.
 2. Thehologram recording medium manufacturing method according to claim 1,wherein in the pattern computation step, a plurality of computationpoints are defined discretely on the recording plane and an amplitudeand a phase of a synthetic object light at a predetermined sampling timepoint is determined for each of computation point positions to determinethe complex amplitude pattern as a discrete distribution of amplitudesand phases.
 3. The hologram recording medium manufacturing methodaccording to claim 2, wherein in the pattern forming step, a cell,formed of a three-dimensional structure, is positioned at eachindividual computation position and information of an amplitude and aphase concerning the computation point position corresponding to eachindividual cell are recorded in the three-dimensional structure of thecell.
 4. The hologram recording medium manufacturing method according toclaim 1, wherein in the original image preparation step, a plurality oforiginal images that are positioned so as to partially overlap spatiallyare prepared.
 5. The hologram recording medium manufacturing methodaccording to claim 1, wherein in the observation region setting step,the plurality N of observation regions are set to be regions that arespatially exclusive with respect to each other.
 6. The hologramrecording medium manufacturing method according to claim 1, wherein inthe observation region setting step, a portion or all of the plurality Nof observation regions are set to be regions that partially overlapspatially with another observation region.
 7. The hologram recordingmedium manufacturing method according to claim 1, wherein in theobservation region setting step, a portion or all of the plurality N ofobservation regions are set to be regions that spatially match anotherobservation region completely.
 8. The hologram recording mediummanufacturing method according to claim 1, wherein point light sourcesor collections of point light sources are used as the unit lightsources, and the object light is defined as a spherical wave that isemitted radially from each point light source or as a synthetic wave ofsuch spherical waves.
 9. The hologram recording medium manufacturingmethod according to claim 1, wherein segment light sources are used asunit light sources, and object light components, each with a wavefrontformed of a side surface of a cylindrical column having a segment lightsource as a central axis, which propagate in a direction perpendicularto the central axis, are defined.
 10. The hologram recording mediummanufacturing method according to claim 1, wherein in the observationregion setting step, each individual observation region is set as aplane, a curved surface, or a three-dimensional body in thethree-dimensional coordinate system.
 11. The hologram recording mediummanufacturing method according to claim 1, wherein in the patterncomputation step, the computation is performed upon partitioning thethree-dimensional space into a plurality M of plate-like spaces byslicing by a plurality of mutually parallel planes and by taking intoaccount only light components, which, among the object light componentsfrom the unit light sources in a j-th (j=1, 2, . . . , M) plate-likespace and belonging to an i-th (i=1, 2, . . . , N) original image, reachan i-th (i=1, 2, . . . , N) observation region and reach the recordingplane only through an interior of the j-th (j=1, 2, . . . , M)plate-like space.
 12. A method for manufacturing a hologram recordingmedium that has an arrangement by which different original images arereproduced when observed from different positions, the hologramrecording medium manufacturing method comprising: an original imagepreparation step of preparing a plurality of original images, each as aset of unit light sources positioned in a three-dimensional coordinatesystem; a recording plane setting step of setting a predeterminedrecording plane in the three-dimensional coordinate system; anobservation region setting step of setting a plurality of observationregions in the three-dimensional coordinate system; a patterncomputation step of computing a complex amplitude pattern formed on therecording plane by synthesis of object light components emitted from theindividual unit light sources constituting the respective originalimages; and a pattern forming step of forming the complex amplitudepattern on a physical medium; wherein one-to-one correspondences aremade between said plurality of original images and said plurality ofobservation regions, respectively; and wherein in the patterncomputation step, a plurality of computation points being defined onsaid recording plane, at each of computation point positions, asynthetic object light is obtained by synthesizing object lights,emitted from the individual unit light sources constituting saidplurality of original images, which reach said each of computation pointpositions to compute a complex amplitude of said synthetic object light,wherein computations that each takes into account only light components,which, among the object light components from the unit light sources,propagate toward a unique observation region set corresponding to anoriginal image to which the unit light sources belong, are performed.13. A hologram recording medium manufactured by the manufacturing methodaccording to claim
 1. 14. A device for manufacturing a hologramrecording medium that has an arrangement by which different originalimages are reproduced when observed from different positions, thehologram recording medium manufacturing device comprising: an originalimage storage unit, storing a plurality N of original images, each asdata indicating a set of unit light sources positioned in athree-dimensional coordinate system; a recording plane setting unit,setting a predetermined recording plane in the three-dimensionalcoordinate system; an observation region setting unit, setting aplurality N of observation regions in the three-dimensional coordinatesystem; a pattern computation unit, computing a complex amplitudepattern formed on the recording plane by synthesis of object lightcomponents emitted from the individual unit light sources constitutingthe respective original images; and a pattern forming unit, forming thecomplex amplitude pattern on a physical medium; wherein the observationregion setting unit sets observation regions so that one-to-onecorrespondences are made between said plurality N of original images andsaid plurality N of observation regions, respectively; and wherein thepattern computation unit defines a plurality of computation points onsaid recording plane and performs computations, at each of computationpoint positions, obtaining a synthetic object light by synthesizingobject lights, emitted from the individual unit light sourcesconstituting said plurality N of original images, which reach said eachof computation point positions to compute a complex amplitude of saidsynthetic object light, wherein computations that each takes intoaccount only light components, which, among the object light componentsemitted from the unit light sources belonging to an i-th (i=1, 2, . . ., N) original image, reach an i-th (1=1, 2, . . . , N) observationregion corresponding to said i-th original image.